Manufacture electrodes for electrochemical monitoring

ABSTRACT

In one aspect, a material system comprises a metal substrate and a first coating layer disposed on the metal substrate. A first electrode is directly disposed on the first coating layer, and a second electrode is disposed on the metal substrate. In one aspect, a method for determining material performance includes flexing a material system and detecting impedance of the material system with an electrochemical impedance spectrometer. The material system has a metal substrate, a first coating layer disposed on the metal substrate, a first electrode directly disposed on the first coating layer, and a second electrode disposed on the metal substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.15/355,604, filed Nov. 18, 2016. The above referenced application isincorporated herein by reference in its entirety.

FIELD

Aspects of the present disclosure generally relate to material systemsand methods for determining operational performance of material systems.

BACKGROUND

Spanning the lifetime of operation, an aircraft will experience repeatedand harsh conditions resulting in degradation of component parts of theaircraft. Such degradation may take the form of, for example, corrosionand subsequent metal fatigue and fracture. Corrosion can contribute to adecrease in the integrity and strength of aircraft components. Morespecifically, a material system, such as an aircraft component, includesa fuselage or skin panels, a coated lap joint between two metal panels,or a wing-to-fuselage assembly on the exterior of an aircraft. Materialsystems may corrode over time due to exposure to mechanical and chemicalstresses during use of the aircraft. Before a material is determined tobe suitable for use as an aircraft material system, it may be desirableto determine the material system's propensity to corrode. However,performance of aircraft material systems, such as panels, during actual,real world use of the aircraft seldom correlates with corrosion testingdata.

Furthermore, a corrosion testing procedure of a material systemcomprises spraying the material system with a salt solution in achamber. Assessment of the extent of corrosion of the material systeminvolves stopping the corrosion procedure and removing the materialsystem from the chamber for visual inspection to determine the extent ofcorrosion.

Therefore, there is a need in the art for material systems, apparatus,and methods for controlled and accurate exposure and corrosion detectionfor determining operational performance of material systems.

SUMMARY

In one aspect, a material system comprises a metal substrate and a firstcoating layer disposed on the metal substrate. A first electrode isdirectly disposed on the first coating layer, and a second electrode isdisposed on the metal substrate.

In another aspect, a method for determining material performancecomprises flexing a material system and detecting impedance of thematerial system with an electrochemical impedance spectrometer. Thematerial system comprises a metal substrate and a first coating layerdisposed on the metal substrate. A first electrode is directly disposedon the first coating layer, and a second electrode is disposed on themetal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toaspects, some of which are illustrated in the appended drawings. It isto be noted, however, that the appended drawings illustrate only typicalaspects of this present disclosure and are therefore not to beconsidered limiting of its scope, for the present disclosure may admitto other equally effective aspects.

FIG. 1 is a top sectional view of an apparatus for accelerating andcontrolling the corrosion-related failure modes of a material system,according to an aspect of the disclosure.

FIG. 2 is a side sectional view of an apparatus for accelerating andcontrolling the corrosion-related failure modes of a material system,according to an aspect of the disclosure.

FIG. 3 is a top sectional view of an apparatus for accelerating andcontrolling the corrosion-related failure modes of a material system,according to an aspect of the disclosure.

FIG. 4 is a perspective view of a flexer configured to perform cyclicflexing, according to an aspect of the disclosure.

FIG. 5 is a side view of a material system, according to an aspect ofthe disclosure.

FIG. 6 is a side view of a material system, according to an aspect ofthe disclosure.

FIG. 7A is a plan view of a material system according to an aspect ofthe disclosure.

FIG. 7B is a plan view of a material system according to an aspect ofthe disclosure.

FIG. 7C is a plan view of a material system according to an aspect ofthe disclosure.

FIG. 8 is a graph of impedance data of a material system comprising aninterdigitated electrode pair of Example 1 disposed onto a coatingmaterial.

FIG. 9 is a graph of impedance data of a material system comprisingelectrodes of Example 2, according to an aspect of the presentdisclosure.

FIG. 10 is a graph of impedance data of a scribed material systemcomprising electrodes of Example 3, according to an aspect of thepresent disclosure.

FIG. 11 is a graph of impedance data of a material system comprisingelectrodes of Example 4, according to an aspect of the presentdisclosure.

FIG. 12 is a graph of impedance data of a material system comprisingelectrodes of Example 5, according to an aspect of the presentdisclosure.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. It is contemplated that elements and features of one aspectmay be beneficially incorporated in other aspects without furtherrecitation.

DETAILED DESCRIPTION

The descriptions of the various aspects of the present disclosure havebeen presented for purposes of illustration, but are not intended to beexhaustive or limited to the aspects disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the described aspects.The terminology used herein was chosen to best explain the principles ofthe aspects, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the aspects disclosed herein.

Aspects of the present disclosure generally relate to material systemsand methods for determining operational performance in situ of materialsystems. A material system can be a component of an aircraft andtypically comprises a substrate, such as a metal, and one or morecoatings, such as an epoxy, disposed on the substrate. One or moreelectrodes, such as a pair of electrodes, are disposed on or within asurface of the material system to provide electrochemical detection ofoperational performance, e.g. corrosion, of the material system.Determining operational performance of a material system can beperformed in a lab setting or on an aircraft by an operator ormanufacturer before, during (in situ), or after the material system hasbeen exposed to flexing and/or moisture treatment. For example, amaterial system is in electrical communication with a spectrometer toprovide impedance data of one or more surfaces of the material system toassist in determination of the operational performance of the materialsystem during flexing and moisture exposure of the material system.

Apparatus

A material system, such as a panel, may have one or more surface layerssuch as a surface finish, a primer, and/or a top coat. Corrosion mayoccur at one or more of these layers in use due to mechanical andchemical stresses. Material systems, apparatus, and methods of thepresent disclosure provide in situ electrochemical monitoring ofimpedance to determine corrosion in a setting that mimics the corrosionexperienced by a material system in actual use conditions. The materialsystem is subjected to mechanical as well as chemical stresses withoutdegradation of the electrochemical monitoring system. Material systems,apparatus and methods of the present disclosure provide electrochemicalmonitoring of impedance to determine corrosion at one or more of amaterial system surface, a finished surface, a primer surface, and/or atop coat surface.

FIG. 1 is a plan view of an apparatus 100 for accelerating andcontrolling the corrosion-related failure modes of a material system,according to an aspect of the disclosure. FIG. 2 is a side perspectiveview of apparatus 100 of FIG. 1. One or more components of apparatus 100are made of materials that show resistance to a corrosive environment,such as an environment containing a salt fog. As shown in FIGS. 1 and 2,apparatus 100 includes an enclosure 160 having one or more fog nozzles102 (one shown) disposed therein and configured to spray a treatingliquid, such as a salt fog, in the enclosure 160. A fixture support isdisposed in the enclosure to support a material system for exposure andflexing therein. Apparatus 100 includes a liquid reservoir 104 to supplya treating liquid to fog nozzle 102. Fog nozzle 102 may be a nozzle,such as an atomizing nozzle, a nozzle calibrated for air consumption,BETE full cone nozzle, hollow cone nozzle, fan misting nozzle, tankwashing spray nozzle, NASA Mod1 nozzle for water spray atomization anddroplet control, Q-Lab OEM fogging nozzle, Cool Clean ChilAire Litespray applicator nozzle, or combinations thereof. Fog nozzle 102 can bemade of materials such as hard rubber, plastic, or other inertmaterials.

The fixture support comprises jaws 124 a-e configured to flex a materialsystem. Plate 146 is configured to support jaws 124 a-e. In at least oneaspect, plate 146 comprises a mounting plate disposed on an I-Beamgrate. Plate 146 is positioned between fog nozzle 102 and jaws 124 a-124e (as shown in FIGS. 1 and 2), allowing treating liquid to enter theenclosure without directly impinging upon a material system held by oneor more jaws 124 a-e. This configuration mimics general humidatmospheric conditions, as compared to direct rainfall onto an aircraftmaterial system. Alternatively, jaws 124 a-e may be positioned betweenfog nozzle 102 and plate 146 (this configuration not shown), providingdirect flow of treating liquid toward a material system held by one ormore jaws 124 a-e. This configuration mimics direct rain fall or aerosoldeposition onto an aircraft material system. Fog nozzle 102 may beconfigured for flow angle adjustment, allowing flow of treating liquidat one or more angles relative to a material system surface. In at leastone aspect, a material system surface may be parallel to a principaldirection of flow of liquid through apparatus 100, based upon thedominant surface being tested, which reduces liquid collection on amaterial system during corrosion testing performed in apparatus 100. Insuch aspects, fog nozzle 102 may be directed or baffled so that theliquid does not impinge directly on a material system. (Fog nozzle 102,a vent 122, a motor 126, an outer enclosure 136, and legs 148 a-f areshown as dashed lines in FIG. 1 to indicate that these parts are locatedbehind a plate 146 in the aspect shown in FIG. 1).

A fog pump 108 is configured to assist flow of a liquid from liquidreservoir 104 to fog nozzle 102 via first fluid line 106 and secondfluid line 110. First fluid line 106 couples liquid reservoir 104 at afirst end with fog pump 108 at a second end to provide liquidcommunication of liquid reservoir 104 with fog pump 108. Second fluidline 110 couples fog pump 108 at a first end with fog nozzle 102 at asecond end to provide liquid communication of fog pump 108 with fognozzle 102.

A compressed air source 112 and bubble tower 114 are configured toprovide humidified air to fog nozzle 102. In at least one aspect, apressure in the enclosure may be regulated to mimic the pressureexperienced by an aircraft at various altitudes during real world use.Accordingly, compressed air source 112 is configured to flow air at apressure ranging from about 2 pounds per square inch (PSI) to about 50PSI, from about 5 PSI to about 30 PSI, from about 12 PSI to about 18PSI. In these ranges, lower pressure values mimic pressures experiencedby an aircraft at higher altitudes while higher pressure values mimicpressures experienced by an aircraft at lower altitudes and closer tosea level. Air may include a mixture of gases similar to that found inan ambient atmosphere, for example, comprising about 78% N₂, about 21%O₂, and about 0.039% CO₂, among other gases. Third fluid line 116couples bubble tower 114 at a first end with fog nozzle 102 at a secondend to provide air and liquid communication of bubble tower 114 with fognozzle 102. A compressed air line 118 couples compressed air source 112at a first end with bubble tower 114 at a second end to provide aircommunication of compressed air source 112 with bubble tower 114. Bubbletower 114 may contain a liquid, such as water, to provide initialhumidification or additional humidification to air flowed fromcompressed air source 112 via compressed air line 118.

A vent 122 may be coupled with the first chamber wall 130, a secondchamber wall 132, or a third wall 152 (FIG. 2) to provide pressureregulation inside of apparatus 100. A heater 120 may be provided andconfigured to heat the inside of apparatus 100 such as enclosure 160.Heater 120 may be disposed adjacent to a first wall 130 of apparatus 100and coupled with third wall 152 (FIG. 2). Heater 120 may be adhered tothird wall 152 by any suitable adherent, such as rivets. Heater 120 maybe coupled with and controlled by controller 138.

Fixture support is configured to support and flex a material systempositioned in the enclosure for testing. Jaws 124 a, 124 b, 124 c, 124d, and 124 e are configured to flex a material system, such as a panel,a coated lap joint between two metal panels, a wing-to-fuselageassembly, or combinations thereof. The material system may be anaircraft material system, such as a panel, such as a skin or fuselageflat panel. The material system may have a width that is, for example,about 4 inches, and a length that is for example, about 6 inches toabout 14.5 inches. The fixture support may flex a material system to astrain ranging from about 0.05% to about 50%, about 0.1% to about 30%,about 0.3% to about 5%, such as about 0.37%.

Fixture support comprising one or more jaws 124 a-e is configured togrip and release a material system. Jaws 124 a-e are configured to flexa material system from a first starting position to a fully or partiallyflexed second position. Jaws 124 a-e are configured to flex a materialsystem from a first position to greater than 0° to about 180° from thestarting position, such as about 5° to about 90°, such as about 5° toabout 45°, during a flexing process. Jaws 124 a-124 e may be the samesize or different sizes. For example, jaw 124 a may be the same size asjaw 124 b, but be a different size than jaw 124 d (as shown in FIG. 1).Furthermore, jaws 124 a-124 e may be positioned from one another by adistance that is the same or different than a distance between adifferent pair of jaws 124 a-e. For example, a first distance betweenjaw 124 a and 124 b may be different than a second distance between jaw124 d and 124 e. Various jaw sizes and various distances between jawsprovide, for example, simultaneous testing of different sized materialsystems, such as panels, during an exposing and flexing process withinapparatus 100. In at least one aspect, one or more of jaws 124 a-ecomprises steel. In at least one aspect, one or more of jaws 124 a-e isanodized. In at least one aspect, one or more of jaws 124 a-e comprisesan inert material such as hard rubber and/or plastic.

In at least one aspect, jaws 124 a-e are configured to support amaterial system, such as a panel, from about 15° and about 30° relativeto a first wall 130 and/or second wall 132, which reduces liquidcollection on a material system during corrosion testing performed inapparatus 100. In at least one aspect, jaw 124 a is configured to grip amaterial system at a first end of the material system and jaw 124 b isconfigured to grip the material system at a second end of the materialsystem. In at least one aspect, jaws 124 a-e are configured to flex amaterial system simultaneously or alternatively.

A motor 126 operates jaws 124 a-e. Inlet tube 128 is coupled with motor126 at a first end and coupled with first wall 130 at a second end forproviding cooling material, such as air, to motor 126. Outlet tube 134is coupled with motor 126 at a first end and coupled with first wall 130at a second end for removing hot air exhaust from motor 126. Outerenclosure 136 surrounds motor 126 to enclose and protect the motor fromliquid emitted from fog nozzle 102 or any other liquid present inside ofapparatus 100. Jaws 124 a-e are supported by plate 146. Plate 146 issupported by legs 148 a, 148 b, 148 c, 148 d, 148 e, and 148 f. Legs 148a-f are coupled with plate 146 at a first end and a chamber wall, a rack150 a, or a rack 150 b at a second end.

Apparatus and material systems of the present disclosure include one ormore electrodes, such as one or more pairs of electrodes. An electrodemay be coupled with a substrate (to form a material system) andsubsequent use of apparatus 100 to test operational performance of thematerial system. During flexing, the center portion of the materialsystem will experience more strain than the edges of the materialsystem. Accordingly, a pair of electrodes disposed on the same side ofthe material system provides detecting impedance across the same side ofthe material system.

As shown in FIGS. 1 and 2, apparatus 100 includes electrode pairs 156and 158. Although electrode pairs are shown in FIGS. 1 and 2, in analternative aspect, apparatus 100 comprises single electrodes. Electrodepair 156 is configured to couple with a first side of a material system(not shown), and electrode pair 158 is configured to couple with asecond side of the material system (not shown). Electrodes can be madeof conductive epoxy, gold, silver, copper, platinum, palladium, ormixtures thereof. Preferably, at least one electrode is conductiveepoxy, such as the electrodes of pairs 156 and/or 158. In at least oneaspect, conductive epoxy is conductive silver epoxy. Electrode pair 156is coupled with spectrometer 164 via electrical line 162 to provideelectrical communication between electrode pair 156 and spectrometer164. Furthermore, electrode pair 158 is coupled with spectrometer 164via electrical line 166 to provide electrical communication betweenelectrode pair 156 and spectrometer 164. Electrical lines 162, 166 canbe insulated wire (e.g., insulated steel wire) or wire having insulatedconductive tape. Electrode pair 156 is configured to couple with a firstside of a material system, and electrode pair 158 is configured tocouple with a second side of the material system, as described in moredetail below. In at least one aspect, spectrometer 164 comprises apotentiostat, galvanostat, and/or zero-resistance ammeter. Spectrometer164 can be an electrochemical impedance spectrometer, such as aReference 600 supplied by Gamry Instruments or a VMP 300 supplied byBio-Logic Science Instruments. When coupled with a material system,electrodes (e.g., electrode pairs 156 and 158) detect an electricalsignal from the material system and transmit the electrical signal to aspectrometer, such as spectrometer 164. Spectrometer 164 is configuredto interpret the electrical signal to provide electrical data, such asimpedance, regarding the condition, such as corrosion, of the materialsystem. Electrochemical impedance is usually measured by applying an ACpotential to an electrochemical cell and then measuring the currentthrough the cell. The response to this [sinusoidal] potential is an ACcurrent signal. This current signal can be analyzed as a sum ofsinusoidal functions (a Fourier series). Electrochemical impedance isnormally measured using a small excitation signal. This is done so thatthe cell's response is pseudo-linear. In a linear (or pseudo-linear)system, the current response to a sinusoidal potential will be asinusoid at the same frequency but shifted in phase. EIS data aretypically analyzed in terms of an equivalent circuit model. EchemAnalyst [a Gamry software product] finds a model whose impedance matchesthe measured data.

Parts of apparatus 100 described herein may comprise materials that aresuitably inert to conditions within apparatus 100 during a cyclicflexing fog spray process. Suitably inert materials may include plastic,glass, stone, metal, rubber, and/or epoxy.

Apparatus 100 may be controlled by a processor based system controllersuch as controller 138. For example, the controller 138 may beconfigured to control apparatus 100 parts and processing parametersassociated with a cyclic flexing fog spray process. The controller 138includes a programmable central processing unit (CPU) 140 that isoperable with a memory 142 and a mass storage device, an input controlunit, and a display unit (not shown), such as power supplies, clocks,cache, input/output (I/O) circuits, and the like, coupled to the variouscomponents of the apparatus 100 to facilitate control of a cyclicflexing fog spray process. Controller 138 may be in electroniccommunication with, for example, outlet tube 134, vent 122, heater 120,and/or jaws 124 a-e.

To facilitate control of the apparatus 100 described above, the CPU 140may be one of any form of general purpose computer processor that can beused in an industrial setting, such as a programmable logic controller(PLC), for controlling various chambers and sub-processors. The memory142 is coupled to the CPU 140 and the memory 142 is non-transitory andmay be one or more of readily available memory such as random accessmemory (RAM), read only memory (ROM), floppy disk drive, hard disk, orany other form of digital storage, local or remote. Support circuits 144are coupled to the CPU 140 for supporting the processor in aconventional manner. Information obtained from cyclic flexing fog sprayprocesses with apparatus 100 may be stored in the memory 142, typicallyas a software routine. The software routine may also be stored and/orexecuted by a second CPU (not shown) that is remotely located from thehardware being controlled by the CPU 140. The memory 142 is in the formof computer-readable storage media that contains instructions, that whenexecuted by the CPU 140, facilitates the operation of the apparatus 100.The instructions in the memory 142 are in the form of a program productsuch as a program that implements the method of the present disclosure.The program code may conform to any one of a number of differentprogramming languages. In at least one aspect, the disclosure may beimplemented as a program product stored on computer-readable storagemedia for use with a computer system. The program(s) of the programproduct define functions of the aspects (including the methods describedherein). Illustrative computer-readable storage media include, but arenot limited to: (i) non-writable storage media (e.g., read-only memorydevices within a computer such as CD-ROM disks readable by a CD-ROMdrive, flash memory, ROM chips or any type of solid-state non-volatilesemiconductor memory) on which information is permanently stored; and(ii) writable storage media (e.g., floppy disks within a diskette driveor hard-disk drive or any type of solid-state random-accesssemiconductor memory) on which alterable information is stored. Suchcomputer-readable storage media, when carrying computer-readableinstructions that direct the functions of the methods and apparatus ofthe present disclosure, are aspects of the present disclosure.

FIG. 3 is a plan view of an apparatus 300 for accelerating andcontrolling the corrosion-related failure modes of a material system,according to an aspect of the disclosure. As shown in FIG. 3, thecomponents of apparatus 300 are the same as the components shown inapparatus 100 of FIG. 1, except that motor 126 is not encompassed byouter enclosure 136 (outer enclosure 136 is not present), motor 126 islocated external to first chamber wall 130, and inlet tube 128, andoutlet tube 134 are not present. Motor 126 is coupled with first chamberwall 130. In at least one aspect, motor 126 (coupled to first chamberwall 130) translates bending motion inside the chamber via a screw, suchas a ball screw, Acme screws, Lead screws, Roller screws, and screwmount, or an axle, passing into the chamber (not shown). A screwmaintains spacing between stationary block 406 and mobile block 404during flexing.

FIG. 4 is a perspective view of a flexer 400 configured to performcyclic flexing, according to an aspect of the disclosure. Flexer 400 maybe located inside of a material performance chamber, such as apparatus100 as described for FIG. 1. As shown in FIG. 4, flexer 400 includes amobile block 404 and a stationary block 406. Stationary block 406 ismounted to plate 146 by mounting bolts 410 a and 410 b. Mobile block 404is slidably disposed on plate 146 adjacent to stationary block 406.Linear displacement between stationary block 406 and mobile block 404 ismaintained by guide rods 408 a and 408 b. Guide rods 408 a and 408 b maycomprise stainless steel, high density polypropylene, high densitypolyethylene, chromium, such as an Armology coating, and combinationsthereof. Each of guide rods 408 a and 408 b is coupled with stationaryblock 406 and mobile block 404. Guide rods 408 a and 408 b are parallelto one another. Stationary block 406 and mobile block 404 mount orotherwise support jaws 124 a-f. Stationary block 406 mounts a first sideof jaws 124 a-f in a stationary position during flexing, while mobileblock 404 mounts a second side of jaws 124 a-f and allows movement ofthe second side of jaws 124 a-f during flexing. Stationary block 406 andmobile block 404 may comprise high density polyethylene. During flexing,the mobile block is shifted laterally relative to the stationary blockfrom a starting point to an end point resulting in flexing of thematerial system positioned in jaws 124 a-f. The starting point, endpoint, and shift distance may be controlled by a user of flexer 400based on test fixture mechanical boundary limits, mechanical stopblocks, or fixture driving system software controls. Each of stationaryblock 406 and mobile block 404 includes a base and a plurality ofstanchions extending from the base perpendicular thereto. Each stanchionmay be rectangular or angled to provide an angle on which each of thejaws 124 a-f can be mounted. Jaws 124 a-f are hinged, which allowsbending of a material system while compressing the material system. Forexample, jaws 124 a-f may be mounted on one side of each stanchion anddisposed at an angle from, for example, 15° to 30° relative to a lineperpendicular to the base. Other angles relative to perpendicular arecontemplated to achieve a desired testing condition for a materialpanel. The angle of jaws 124 a-f determines the angular position of thematerial system. In at least one aspect, a material system is disposedat an angle from, for example, 15° to 30° relative to a lineperpendicular to the base. In at least one aspect, jaws 124 a-f arenon-conductive and non-metallic so as to have little or no galvaniceffect on the material system. One or more of jaws 124 a-f may comprisehigh density polyethylene, commercial grade Titanium (II) withpolyethylene insert, sacrificial 316SS with polyethylene insert, orcombinations thereof, which prevents (partial or complete) galvaniccorrosion of the jaws and material systems during testing. One or moreof jaws 124 a-f may comprise a sleeve cover comprising, for example,polyethylene, which further prevents galvanic corrosion of the jaws andmaterial systems during testing. Electrode pair 158 of apparatus 100 isdirectly disposed on a first side of a material system disposed in jaw124 f, and electrode pair 156 (not shown) of apparatus 100 is disposedon a second side of the material system opposite the first side. Anon-conductive protective coating 412 (shown as a transparent coatingfor simplicity) is disposed on electrode pairs 158 and 156 to protectthe electrodes from a corrosive environment while apparatus 100 is inuse, e.g. testing the material system. Non-conductive protectivecoatings include non-conductive epoxy, tape, adhesive, sealant, ormixtures thereof. In at least one aspect, non-conductive epoxy is a2-part waterproof epoxy. Although FIG. 4 shows electrodes directlydisposed on a material system disposed in jaw 124 f, it is to beunderstood that material systems (and apparatus) of the presentdisclosure embrace aspects where one or more electrodes and/or electrodepairs are directly disposed on material systems disposed in one or moreof jaws 124 a-e, and the electrodes and/or electrode pairs can be inelectrical communication with spectrometer 164 via electrical linessimilar or identical to electrical lines 162, 166. Such aspects provideelectrochemical in situ monitoring of a plurality of material systemswithin a material performance chamber, such as apparatus 100.Furthermore, a non-conductive protective coating, such as coating 412,can be disposed on said electrodes and/or electrode pairs.

In at least one aspect of the present disclosure, a material performancechamber contains more than one flexer 400. In at least one aspect wherea material performance chamber contains more than one flexer 400, guiderods 408 a and 408 b extend through multiple flexers 400.

A flexer, such as flexer 400, provides variable displacement of a mobileblock and material systems at variable frequencies that are adjustablein real-time. A flexer also provides for application of tension andcompression to a material system.

Material Systems

In at least one aspect, a material system is a metal panel that can beflat and can be coated. The material performance of the flat panel istested by cyclically flexing the material system while exposing thepanel to at least a cycle of salt fog. Before, during, and/or afterexposure and flexing, the material system is assessed for corrosiononset, rate of propagation, and performance.

In at least one aspect, a material system comprises a substrate havingtwo flat metal panels connected, joined, welded, bonded, or fastenedtogether using metallic fasteners, screws, bolts, or other hardware,before being exposed to at least a cycle of salt fog.

In at least one aspect, a material system comprises a mechanical jointor knuckle joint that may be made of metallic or composite materials andcoated before being exposed to a cyclic salt fog and/or before beingassessed for corrosion onset, rate of propagation, and performance.

In at least one aspect, a material system comprises a structural systemreplicative of aircraft components, representing a side-of-body joint, astringer-to-fuselage assembly, a fuselage panel, or wingspar-to-fuselage assembly. The produced assemblies may be actuated orflexed while being exposed to at least a cycle of salt fog before/whilebeing assessed for corrosion onset, rate of propagation, andperformance, as described herein.

FIG. 5 is a side view of a material system 500 depicting material system500 comprising a conductive metal substrate 502 and a coating layer 504disposed on substrate 502. Metal substrate 502 can be made of titanium,aluminum, copper, or alloys thereof. Metal substrate 502 may be coatedwith one or more primers, such as a chromated primer, surface finishesand/or top coats. For example, coating layer 504 can be made ofchromated primer, epoxy primer, urethane primer, or mixtures thereof.Electrode 506 (which can be part of an electrode pair such as electrodepair 158) is directly disposed on coating layer 504 and is a referenceelectrode. Electrode 508 (which can be part of an electrode pair such aselectrode pair 156) is disposed on the metal substrate 502 and is aworking electrode. In at least one aspect, an insulating adhesive, suchas non-conductive epoxy, is disposed between electrode 508 and metalsubstrate 502. For spectroscopic measurements during testing, theworking electrode 508 is adhered to conductive metal substrate 502, andan electrical signal is sent through the working electrode (or pair ofelectrodes). The signal then moves through coating layer 504 and isreceived by electrode 506 (of electrode pair 158) and transmitted tospectrometer 164. In an alternative aspect, working electrode 508 andreference electrode 506 is each disposed (e.g., directly disposed) oncoating layer 504.

In aspects where a coating layer, such as coating layer 504, is made ofan epoxy and an electrode disposed on the coating layer is made of aconductive epoxy, it has been discovered that the epoxy materials of thecoating layer and the electrode absorb to one another. Use of anadhesive to adhere the two materials together is optional such that theelectrode is directly disposed on the coating layer. In such aspects, asurface of the coating layer can be lightly abraded, followed byapplying the electrode directly to the abraded surface. This“like-on-like” interaction between coating layer and electrode improvescompatibility of the interface of the electrode and coating layer. Theimproved compatibility between the electrode and coating layer improvesthermal and mechanical properties between the coating layer and theelectrode. Conventional electrodes are adhered to a substrate surfacewith non-conductive adhesives. These adhesives interfere with theelectrical communication of the electrode and a substrate such as acoating layer, yielding inaccurate spectroscopic data. With use of suchadhesives, the electrical properties of the material system are beingaffected by a component (the adhesive) that is not a component of amaterial system that would be used in commercial applications. Theadhesive causes a sharp gradient in mechanical, chemical, and thermalperformances of the material system where the electrode is located. Theimproved compatibility between electrodes and coating layers of materialsystems of the present disclosure provides homogeneity between theelectrodes and coating layers yielding reduced noise observed in aspectroscopic signal.

As a comparative example to material systems having epoxy electrodes, amaterial system having metal electrodes deposited onto a coating layerwas tested. Electrochemical monitoring of the material system havingmetal electrodes deposited onto a coating layer provided an EIS spectrumshowing only an “air” curve, indicative of an insufficient interactionbetween the metal electrodes and the coating layer. As used herein, “aircurve” indicates an open-lead experiment. This experiment records an EISspectrum with no cell attached. The spectrum from an open-leadexperiment looks very much like a noisy spectrum for a parallel RCnetwork. So, when an air curve is observed in the data, the leads fromthe spectrometer are not making electrical contact with the coating, andan EIS spectrum of the open air (i.e. an “air curve) is being collected.

Furthermore, it has been discovered that the thickness of electrodes ofa material system can affect spectroscopic results of electrochemicalmonitoring. Electrodes of the present disclosure, such as electrodes 506and 508 of electrode pairs 156 and/or 158, can have a thickness of about12 micrometers (μm) or less. Electrodes having a thickness of about 12μm or less provide flexibility of the electrodes disposed on and/orwithin a material system and provide material systems operable to havean electrode disposed on one or more layers of the material system formore accurate electrochemical monitoring of each of the one or morelayers of a material system. In at least one aspect, electrodes of thepresent disclosure have a thickness of from about 1 μm to about 12 μm,such as from about 2 μm to about 11 μm, such as from about 3 μm to about10 μm. In at least one aspect, a coating layer of the present disclosurehas a thickness of from about 1 μm to about 500 μm, such as from about 2μm to about 250 μm, such as from about 3 μm to about 100 μm, such asfrom about 4 μm to about 15 μm. Furthermore, the reduced size of theelectrodes of the present disclosure provides smaller/thinner electricalwires (coupled with the electrodes at a first end and a spectrometer ata second end) to be used for material systems of the present disclosure,as compared to traditional electrical wires that are too large to beembedded within layers of a multilayered material system.

In comparison, an electrode having a thickness of 13 μm or greater (suchas interdigitated electrodes) is more rigid than thinner electrodes andtends to disconnect from the material system during flex testing. Therigidity of thick electrodes hinders the electrode's ability to conformto a surface of the material system. Furthermore, if a conventionallythin coating layer (such as an assembly primer, interior primer, fueltank primer) is disposed on an electrode, electrodes having a thicknessof 13 μm or greater tend to create a defect in the overlying layer andthe defect is then accentuated over the course of flex testing.Furthermore, some conventional electrode designs involve drillingthrough the substrate to embed electrodes within a layer. Such embeddedelectrodes have similar drawbacks as described for thick electrodes.

In at least one aspect, electrodes of a material system of the presentdisclosure are offset from one another. For example, as shown in FIG. 5,electrodes 506 and 508 are offset from one another by a distance (d).Offsetting the electrodes of material systems of the present disclosurereduces moisture effects because an electrical signal flows where theelectrons have the least resistance. If the electrodes are not offsetfrom one another, then the area under the reference electrode isshielded from absorbing electrolyte from moisture. As moisture contentwithin a coating increases (e.g., in the cracks/crevices) duringtesting, the accuracy of electrical data is improved because of therelatively high dielectric constant of water and saline as compared tothe dielectric constant of most intact coatings. Preferably, theelectrodes themselves are protected from moisture or the electricalsignal may be inaccurate. Protecting an electrode from moisture may beaccomplished by sealing an electrode with a protective coating, such asa non-conductive epoxy.

FIG. 6 is a side view of a material system 600, according to an aspectof the present disclosure. As shown in FIG. 6, material system 600 is amultilayered material system comprising metal substrate 502, a firstcoating layer 602, and a second coating layer 504. Electrode 608 (whichcan be of an electrode pair) is disposed on metal substrate 502 and isin electrical communication with a spectrometer, such as spectrometer164, via electrical line 610. Furthermore, electrode 604 (which can beof an electrode pair) is disposed on first coating layer 602 and is inelectrical communication with a spectrometer, such as spectrometer 164,via electrical line 606. A protective coating (not shown), such ascoating 412, can be disposed on one or both of electrodes 608 and 604before depositing a subsequent coating layer onto the electrodes andsubstrate. As shown in FIG. 6, electrodes 608 and 604 are internal to(e.g., embedded) the material system. Internal electrodes provide insitu electrochemical monitoring of individual layers of a materialsystem at a coating/substrate interface of a multilayered materialsystem to determine corrosion. In at least one aspect, an insulatingadhesive, such as non-conductive epoxy, is disposed between electrode608 and metal substrate 502.

As shown in FIG. 6, electrode 608 and electrode 508 are offset by adistance (d₁). Electrode 508 and 506 are offset by a distance (d₂).Electrode 506 and electrode 604 are offset by a distance (d₃). (d₁),(d₂), and (d₃) are sized to prevent polarizing an electrode, which wouldotherwise move away from the pseudo-linear portion of a voltage-currentresponse curve. In at least one aspect, (d₁)=(d₂)=(d₃). In at least oneaspect, (d₁), (d₂), and/or (d₃) is between about 0.3 cm and about 10 cm,such as between about 0.5 cm and about 3 cm, for example about 1 cm.

Furthermore, varying the surface area of a surface of an electrode thatcontacts an underlying surface affects the electrochemical interactionof the electrode with the underlying surface. One way to take advantageof varying the surface area for a desired application is to vary theshape of one or more electrodes because, other parameters being equal,different shapes result in different surface areas of a contact surfaceof the electrode, as explained in more detail below. Electrodes ofmaterial systems of the present disclosure can have a variety of shapes.For example, an electrode of the present disclosure is square shaped.Alternatively, an electrode of the present disclosure has a shapeselected from circular, star, rectangular, or polygonal, such aspentagonal, hexagonal, heptagonal, or octagonal. Furthermore, electrodesof the present disclosure may have one or more spokes extending (e.g.,outwardly) from the shape.

An electrode of the present disclosure has a surface area (includingspokes if present) that contacts an underlying layer (i.e., a contactsurface area) that is suitable for a desired application. In at leastone aspect, an electrode has a contact surface area from about 0.2 cm²to about 10 cm², such as from about 0.5 cm² to about 5 cm², such as fromabout 1 cm² to about 2 cm². The overall shape, spokes, and surface areacan affect electrochemical monitoring methods for a particular testingapplication of the present disclosure.

Each of FIGS. 7A, 7B, and 7C is a plan view of a material systemaccording to an aspect of the present disclosure. As shown in FIG. 7A,material system 500 (of FIG. 5) comprises electrode pair 158 comprisingelectrodes 506 having a circular shape. Protective coating 412 isdisposed on electrodes 506. In at least one aspect, protective coating412 is also disposed on electrical wire 166 (not shown) to furtherprotect wire 166 during flexing and/or salt fog exposure. As shown inFIG. 7B, material system 700 comprises electrode pair 706 comprisingelectrodes 702 having a rectangular shape. Each of electrodes 702 isdisposed on material layer 704. Protective coating 720 (shown astransparent for clarity) is disposed on electrodes 702. In at least oneaspect, protective coating 720 is also disposed on electrical wire 708(not shown) to further protect wire 708 during flexing and/or salt fogexposure. Each of electrodes 702 can be in electrical communication witha spectrometer via electrical line 708. As shown in FIG. 7C, materialsystem 710 comprises electrode pair 714 comprising electrodes 712 havinga pentagonal shape. Each of electrodes 712 has five spokes 712 aextending outwardly from the pentagonal shape of electrodes 712. Each ofelectrodes 712 is disposed on material layer 718. Protective coating 722(shown as transparent for clarity) is disposed on electrodes 712/712 a.In at least one aspect, protective coating 722 is also disposed onelectrical wire 716 (not shown) to further protect wire 708 duringflexing and/or salt fog exposure. Electrodes 702 can be in electricalcommunication with a spectrometer via electrical line 716.

Fabricating Material Systems

Fabricating a material system of the present disclosure can includelightly abrading an area of a coating layer that the electrode will beapplied to. The abraded area can be cleaned with any suitable solventand allowed to dry. Fabricating further includes disposing an electrodeonto a coating layer, such as an abraded area of the coating layer. Anend portion of insulation of an electrical wire, such as wire 166, canbe removed to form an exposed portion of the electrical wire. Theexposed portion is then contacted with an electrode, followed byapplication of non-conductive tape and/or a protective coating, such asprotective coating 412.

Electrodes (and coating layers) of the present disclosure may bedisposed on a metal substrate or layer by any suitable depositionprocess. Deposition processes include screen printing and 3D printing.In addition, photolithography may be applied to a coating layer followedby deposition of an electrode into the photolithographed region of thelayer.

An electrode, for example, may be deposited using any suitable screenprinting apparatus supplied, for example, by ASM Assembly Systems ofMunich, Germany. Screen printing can be performed using a screen havingone or more openings shaped with the desired geometry for electrodeformation. A deposition material may be placed onto a portion of thescreen and then squeegeed across the opening with a squeegee. Morespecifically, the screen is located over and just above the surface tobe printed so that ink can be accurately deposited in the desiredposition. The mesh of the screen is brought into contact with thesurface by the squeegee as it is moved across the screen. Ink is pushedinto the open area forming the pattern and the surplus is removed by theedge of the squeegee. The mesh should peel away from the surfaceimmediately behind the squeegee, leaving all the ink that was in themesh deposited on the printing surface. The screen can then be liftedclear. The recommended screen tension is the tension necessary tostretch the mesh sufficiently to cause the screen to peel away from thesubstrate after printing but not be stretched so much that damageoccurs. The applied tension depends on the screen material, e.g. theextension used for nylon meshes is typically 6% and for polyester 3%. Itis normal practice for the squeegee to be held at a 45° angle relativeto the frame area.

An electrode, for example, may be deposited using any suitable 3Dprinting apparatus supplied, for example, by nScrypt, Inc. of Orlando,Fla. The nScript apparatus dispenses a conductive ink, e.g. DuPont CB230silver-coated copper conductive ink or DuPont CB028 flexible silver ink,at a material flow rate that is adjusted by backpressure on the nozzle.The speed of the nozzle movement while patterning is constant, and thebackpressure of the material in the nozzle is directly proportional tothe flow rate. The nScrypt printing apparatus has a range ofbackpressures from 0 psi to about 30 psi. For the deposition ofconductive ink onto coated panels, 18 psi backpressure can be used,which corresponds to a flow rate of about 0.052 grams/minute. Afterdeposition of electrodes with the nScript apparatus, ink is baked for afixed time at an elevated temperature to facilitate curing, e.g. 170° C.for 30 minutes.

A coating layer, for example, may be photolithographed using anysuitable photolithography apparatus. Electrodes formed byphotolithography are typically interdigitated electrodes.

Suitable interdigitated electrodes can be obtained from, for example,Synkera Technologies, Inc. of Longmont, Colo. or Micrux Technologies,S.L. of Oviedo, Spain.

Testing Methods

A material testing process such as a cyclic flexing fog spray process,for example, within apparatus 100, may be performed by exposing amaterial system, such as a panel, to a treating liquid, such as a saltfog, and flexing the material system. The exposing may be performed forfrom about 1 hour to about 4500 hours, such as about 200 hours to about2000 hours, such as about 500 hours to about 1000 hours. Exposing amaterial system to a treating liquid for about 1 hour mimics, forexample, salt fog exposure experienced by the material system as part ofan aircraft in an arid climate. Exposing a material system to a treatingliquid for about 4500 hours mimics, for example, salt fog exposureexperienced by the material system as part of an aircraft in a veryhumid climate or a moderately humid climate for a prolonged period oftime. The liquid may contain water that is reagent grade water. Theliquid may be a salt solution. The salt solution may comprise sodiumchloride. The salt solution may contain about 2 parts sodium chloride in98 parts water to about 6 parts sodium chloride in 94 parts water, suchas about 5 parts sodium chloride in about 95 parts water. The liquid,such as a salt solution, may contain less than about 0.1% of bromide,fluoride and iodide. The liquid, such as a salt solution, may containless than about 1 ppm, such as about 0.3 ppm, by mass of copper. Theliquid, such as a salt solution, might not contain anti-caking agents,as such agents may act as corrosion inhibitors. Material systems whichmay be tested include, for example, aircraft panels which may form theskins or fuselage of an aircraft, a coated lap joint between two metalpanels, a wing-to-fuselage assembly, and combinations thereof. Theliquid may be atomized to form the treating liquid, such as a salt fog,that may have a pH ranging from about 3 to about 11, such as about 5 toabout 8, such as about 6.5 to about 7.2. pH may be measured using asuitable glass pH-sensing electrode, reference electrode, and pH metersystem. It may be desirable to adjust the pH of the treating liquid. Forexample, a treating liquid having a low pH may mimic a pollutedatmosphere containing acid rain and the like. Furthermore, pH of theliquid that is atomized into the treating liquid may be adjusted torecalibrate the liquid during an exposing process. pH may be adjustedby, for example, addition of hydrochloric acid (HCl) to decrease the pHor addition of sodium hydroxide (NaOH) to increase the pH. The liquid,such as a salt fog, may be flowed at a rate of about 0.5 milliliters perhour (mL/h) to about 5 mL/h per 80 cm² of horizontal collection area,such as about 1 mL/h to about 2 mL/h per 80 cm² of horizontal collectionarea. In at least one aspect, a material system, such as a panel, may beflexed by a fixture support using one of jaws 124 a-e or by a pluralityof jaws 124 a-e. Flexing may be performed at varying frequencies tomimic the effect of mechanical stresses for corrosive conditionsexperienced by an aircraft material system under real world conditions.For example, a material system may be flexed at a frequency from about0.1 Hertz (Hz) to about 150 Hz, about 0.1 Hz to about 100 Hz, about 0.1Hz to about 60 Hz. Furthermore, the greater the curvature of a flexedmaterial system, the greater the degradation to the material systemusing apparatus and methods of the present disclosure. For example, aflat panel having a length of 6 inches may be gripped by two jaws with adistance of 6 inches between the two jaws. The panel may be flexed at arate of 0.33 Hz during exposure to a salt fog solution. In anotherexample, a flat panel having a length of 7.5 inches may be gripped bytwo jaws also having a distance of 6 inches between the two jaws. Thepanel may be flexed at a rate of 0.33 Hz during exposure to a salt fogsolution. The panel having a length of 7.5 inches has an increasedcurvature and undergoes increased degradation as compared to the panelhaving a length of 6 inches under otherwise identical conditions.Without being bound by theory, mechanical stresses that give curvatureto a material system result in cracking of the material system whichpermits access of corrosive fluid, such as a salt fog, into a crack ofthe material system. After entering a crack of the material system,corrosive fluid may further enter between various additional layers(such as an underlying coating layer), if present. Accordingly,corrosive fluid may cause corrosion of the material system and/or one ormore of the additional layers of the material system. Such conditionsmimic the conditions experienced by an aircraft material system, such asa panel, during real world use.

In at least one aspect, an exposure zone, such as an enclosure 160 ofapparatus 100, may be maintained at a temperature ranging from about−196° C. to about 100° C., −50° C. to about 95° C., 0° C. to about 50°C., such as about 33° C. to about 37° C., for example about 35° C.,during the exposing of a material system to a treating liquid (such as asalt solution atomized into a salt fog), and/or the flexing the materialsystem. The temperature may be monitored by a recording device or by athermometer (not shown) that can be read from an outside surface ofapparatus 100. In at least one aspect, exposing a material system, suchas a panel, to a liquid, such as a salt fog, and flexing the materialsystem may be performed concurrently. In at least one aspect, exposing amaterial system, such as a panel, to a liquid, such as a salt fog, andflexing the material system may be performed sequentially. In at leastone aspect, a material system may be exposed to a salt fog and flexedconcurrently as well as sequentially, which provides recreation of anirregular or variable flight-specific strain profile that may beexperienced by a material system in service. In at least one aspect,exposing a material system to a liquid and/or flexing the materialsystem may be interrupted to visually inspect, rearrange, or remove thematerial system, and/or replenish a solution, such as a solution inliquid reservoir 104.

Before, during (in situ), and/or after flexing and spraying, theimpedance of one or more layers of the material system can be measuredusing an electrochemical impedance spectrometer. Electrochemicalimpedance spectroscopy (EIS) provides in situ measurements of impedanceof one or more layers of the material system. The measurements provideinformation for determining coating properties, such as coatingdegradation, corrosion at the substrate/coating interface, and absorbedmoisture over a period of time. Electrochemical impedance spectroscopicprocesses of the present disclosure can be performed at an excitationpotential of from about 5 mV to about 150 mV, such as about 10 mV toabout 20 mV. Electrical frequencies for EIS may be from about 0.1Hz-10,000 Hz, such as from about 1 Hz to about 5,000 Hz, such as fromabout 1 Hz to about 100 Hz, such as about 0.01 Hz to about 10 Hz, orfrom about 100 Hz to about 4,000 Hz. In at least one aspect, EIS isperformed continuously at a set interval and fixed frequency from about0.5 Hz to about 100 Hz, such as from about 1 Hz to about 10 Hz.

In the following examples, a material system measuring 3.75 inches wideby 14.5 inches long was secured by two jaws in a fixture support in thedevice described in FIG. 1. While flexing the panel at about 1 Hz, thepanel was exposed to a sodium chloride salt fog (pH 6.8) for severaldays.

Example 1: Material System Having Conventional Interdigitated Electrodes

Using conventional interdigitated electrodes, resistance is typicallymeasured between the metal interdigitated electrode and underlyingsubstrate as they corrode. In that case, thicker electrodes work betterbecause the electrode is corroded during the process. If the electrodeis too thin, the electrode will corrode away over time during testing.

For this example, interdigitated electrodes were secured to a topsurface and bottom surface of a coated aluminum coupon using doublesided tape. The electrodes were masked with plater's tape. A chromatedprimer coating was applied to the coupon (including theelectrode-areas). The plater's tape was removed once the coating hadcured. Wires were soldered to the electrodes. The electrodes and wirewere then insulated with a 2-part epoxy and allowed time to cure to formthe completed material system. Corrosion testing was performed within acyclic corrosion chamber as described above using ASTM B117. EIS wasperformed at 150 mV excitation potential, 10 Hz-10,000 Hz frequencyrange, and was performed continuously at a set interval and fixedfrequency of 1 Hz.

FIG. 8 is a graph of impedance data of a material system comprisinginterdigitated electrodes of Example 1. The material system was exposedto salt fog for 5 days. As shown in FIG. 8, impedance decreases overtime upon moisture ingress into the material system. It was observedthat the interdigitated electrodes cause a defect zone in the coating.Furthermore, the values of the data observed are indicative of theactual impedance of only the coating disposed over the electrodes, areasurrounding the electrodes and coupon (due to the application of doublesided tape for adhering the electrodes to the coupon).

Example 2: Material System Having Thin Circular Electrodes Formed by aDispensing

Gun

A bottom and top surface of a coated aluminum coupon was lightlyabraded, and cleaned with a solvent and allowed to dry. Electrodes wereformed on the abraded areas using a conductive silver epoxy to athickness of less than about 12 μm that was applied to a surface using adispensing gun. Each electrode had a contact surface area of 1 cm² and acircular shape. Approximately 0.5 cm of insulation was stripped from athin gauge wire, and the wire was taped to the coupon so that theexposed piece of wire was laying flat across the electrode applicationsite. A 2-part waterproof epoxy was used as a protective coating toelectrically and physically isolate the electrodes from the environmentto form the completed material system. Corrosion testing was performedwithin a cyclic corrosion chamber as described above using ASTM B117.EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hzfrequency range, and was performed continuously at a set interval andfixed frequency of 1 Hz.

FIG. 9 is a graph of impedance data of a material system comprising thinelectrodes of Example 2, according to an aspect of the presentdisclosure. The material system was exposed to salt fog for 4 days. Asshown in FIG. 9, impedance decreases over time upon moisture ingressinto the material system. Unlike coupons having conventionalinterdigitated electrodes, the electrodes did not release from thecoupon surface in the absence of adhesive during testing and did notsubstantially corrode during testing.

Example 3: Scribed Material System Having Thin Circular ElectrodesFormed by a

Dispensing Gun

A bottom and top surface of a scribed coated aluminum coupon was lightlyabraded and cleaned with a solvent and allowed to dry. Electrodes wereformed on the abraded areas to a thickness of less than about 12 μmusing a conductive silver epoxy that was applied to a surface using adispensing gun. Each electrode had a contact surface area of 1 cm² and acircular shape. Approximately 0.5 cm of insulation was stripped from athin gauge wire, and the wire was taped to the coupon so that theexposed piece of wire was laying flat across the electrode applicationsite. A 2-part waterproof epoxy was used as a protective coating toelectrically and physically isolate the electrodes from the environmentto form the completed material system. Corrosion testing was performedwithin a cyclic corrosion chamber as described above using ASTM B117.EIS was performed at 150 mV excitation potential, 0.1 Hz-10,000 Hzfrequency range, and was performed continuously at a set interval andfixed frequency of 1 Hz.

FIG. 10 is a graph of impedance data of a scribed material systemcomprising thin electrodes of Example 3, according to an aspect of thepresent disclosure. The material system was exposed to salt fog for 4days. As shown in FIG. 10, impedance decreases over time upon moistureingress into the material system. Unlike coupons having conventionalinterdigitated electrodes, the electrodes did not release from thecoupon surface in the absence of adhesive during testing and did notsubstantially corrode during testing.

Example 4: Material System Having Embedded Thin Rectangular ElectrodesFormed by Screen Printing

Rectangular electrodes were screen printed to a thickness of less thanabout 12 μm onto a top surface and bottom surface of an anodized andpainted aluminum coupon. The electrodes were fabricated from Ag-530 inkmanufactured by Conductive Compounds. Wire leads were soldered to theelectrodes, and the electrodes and wire leads were sealed withnon-conductive epoxy. A boric sulfuric acid anodized (BSAA) primercoating was applied to the coupon (including the electrode areas) toform the completed material system. Corrosion testing was performedwithin a cyclic corrosion chamber as described above using ASTM B117.EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hzfrequency range, and was performed continuously at a set interval andfixed frequency of 1 Hz.

FIG. 11 is a graph of impedance data of a material system comprisingthin electrodes of Example 4, according to an aspect of the presentdisclosure. The material system was exposed to salt fog for 5 days. Flatcell (FC) measurements and sensor (S) measurements were performed. Flatcell measurements involve the use of a specialized electrochemical cellfilled with salt solution. This creates a “bulk electrolyte” on top ofthe coating. With the embedded sensor measurements, we are conductingEIS in a salt spray chamber which mimics atmospheric exposures. Salt fogleaves thin films of electrolyte on the coated surface. Thin films andbulk electrolytes have different diffusion properties, which can impactcorrosion kinetics and absorption of moisture into the coatings. Thiscan cause slight differences in the EIS spectrum. Thus, the EIS spectrafrom the sensors are compared to those in a flat cell because flat celltechniques are standardized. The feasibility and novelty of performingEIS is demonstrated with embedded electrodes by showing that thesesensors give nearly-equivalent data as a flat cell experiment withouthaving to use a specialized test cell or take the articles out of thechamber for analysis.

As shown in FIG. 11, impedance decreases over time upon moisture ingressinto the material system. Unlike coupons having conventionalinterdigitated electrodes, a defect zone in the coating was not formed(1) in a material system having thin electrodes formed by screenprinting and (2) in the absence of adhesive between the electrodes andthe coupon surface.

Example 5: Material System Having Thin Electrodes Formed by 3D Printing

A chromated primer coating was applied to a coupon, followed by an epoxycoating deposited on the chromated primer. The epoxy coated aluminumcoupon was not abraded, and two electrodes were disposed onto a topsurface of a coupon and two electrodes were disposed onto a bottomsurface of the coupon, each electrode disposed to a thickness of lessthan about 12 μm using an nScrypt 3D printer. Electrode material wasDupont CB230 ink, which is a silver coated copper conductive material.Wire leads were soldered to the electrodes. The electrodes and leadswere sealed with non-conductive epoxy to form the completed materialsystem. Corrosion testing was performed within a cyclic corrosionchamber as described above using ASTM B117. EIS was performed at 150 mVexcitation potential, 10 Hz-10,000 Hz frequency range, and was performedcontinuously at a set interval and fixed frequency of 1 Hz.

FIG. 12 is a graph of impedance data of a material system comprisingthin electrodes of Example 5, according to an aspect of the presentdisclosure. The material system was exposed to salt fog for 5 days. Flatcell (C4 FC) measurements and sensor (C4 S) measurements were performed.As shown in FIG. 12, impedance decreases over time upon moisture ingressinto the material system. Unlike coupons having conventionalinterdigitated electrodes, the electrodes did not release from thecoupon surface in the absence of adhesive during testing and did notsubstantially corrode during testing. Unlike coupons having conventionalinterdigitated electrodes, a defect zone in the coating was not formed(1) in a material system having thin electrodes formed by 3D printingand (2) in the absence of adhesive between the electrodes and the couponsurface.

Material systems, apparatus and methods of the present disclosureprovide a controlled salt fog environment and monitoring of materialperformance, such as corrosion, on a variety of material systems, suchas aircraft material systems, such as panels, coated lap joints betweentwo or more panels, wing-to-fuselage assemblies, or combinationsthereof. Material systems, apparatus and methods of the presentdisclosure provide an ability to replicate in-service, real-worldfailure modes and mechanisms in a controlled exposure environment.

Mechanical flexing of a material system in an apparatus of the presentdisclosure may result in increased corrosion of a material system. Thecompounding effects of mechanical and chemical stresses combine toinduce corrosion, which more accurately replicates corrosion experiencedby a material system, such as an aircraft panel, in a real-worldenvironment. Accordingly, material systems, methods and apparatus of thepresent disclosure more accurately simulate the corrosion observed withaircraft material systems during real-world use of the aircraft.Material systems, methods and apparatus of the present disclosure allowfor testing corrosion of stand-alone material systems and the interfacesbetween coating layers, which more accurately represents the corrosionexperienced by material systems, such as panels, during actual use ofthe material systems as part of an aircraft. Material systems, methodsand apparatus of the present disclosure further provide re-creation ofirregular flight-specific strain profiles so that improved predictive aswell as forensic investigations of aircraft material systems may beperformed.

Material systems, methods and apparatus of the present disclosureprovide electrochemical monitoring of a coating during outdoor exposure,accelerated testing in an environmental chamber, and electrochemicalmonitoring of material systems while in use (e.g, in situ). In situelectrochemical monitoring provides assessment of the integrity of amaterial system without visual inspection of the material system anddoes not require stoppage of a flexing and/or salt fog exposure of thematerial system. Material systems of the present disclosure furtherprovide thin electrodes which can be located on an outer surface of amaterial system or embedded within the material system (e.g., disposedbetween two layers). Such material systems reduce or eliminate defectzones in a coating disposed on the electrode-area of the materialsystem. Furthermore, material systems of the present disclosure furtherprovide reduced or eliminated adhesive use between the electrode and anunderlying substrate which provides accurate electrochemical data from aspectrometer during testing. Material systems of the present disclosurefurther provide electrodes with reduced or eliminated corrosion duringtesting. Material systems of the present disclosure further provideelectrodes that can be shaped to provide a controllable contact surfacearea for desired electrochemical applications.

While the foregoing is directed to aspects of the present disclosure,other and further aspects of the present disclosure may be devisedwithout departing from the basic scope thereof. Furthermore, while theforegoing is directed to material systems, such as aircraft materialsystems, such as panels, coated lap joints between two or more panels,and wing-to-fuselage assemblies, aspects of the present disclosure maybe directed to other material systems not associated with an aircraft,such as a multicomponent material system used in aerospace, automotive,marine, energy industry, and the like.

What is claimed is:
 1. A material system comprising: a metal substrate;a first coating layer disposed on the metal substrate; a first electrodedisposed directly on the first coating layer; and a second electrodedisposed on the metal substrate or the first coating layer, wherein thefirst electrode or the second electrode has one or more spokes extendingtherefrom.
 2. The material system of claim 1, wherein the metalsubstrate comprises titanium, aluminum, copper, or alloys thereof. 3.The material system of claim 1, wherein the coating layer comprises anepoxy.
 4. The material system of claim 1, wherein the first electrode orsecond electrode comprises a conductive epoxy.
 5. The material system ofclaim 1, wherein the first electrode and the second electrode each has athickness from about 1 μm to about 12 μm.
 6. The material system ofclaim 5, further comprising a second coating layer disposed on the firstelectrode and the first coating layer such that the first electrode isembedded between the first coating layer and the second coating layer.7. The material system of claim 6, further comprising a third electrodedirectly disposed on the second coating layer.
 8. The material system ofclaim 6, wherein the second coating layer has a thickness from about 4μm to about 15 μm.
 9. The material system of claim 1, wherein the firstelectrode or the second electrode has a shape selected from the groupconsisting of circular, star, rectangular, pentagonal, hexagonal,heptagonal, and octagonal.
 10. A material system comprising: a metalsubstrate; a first coating layer disposed on the metal substrate; afirst electrode disposed directly on the first coating layer; and asecond electrode disposed on the metal substrate or the first coatinglayer, wherein: the first electrode or the second electrode has one ormore spokes extending therefrom, the first electrode and the secondelectrode each has a thickness from about 1 μm to about 12 μm, and thecoating layer comprises an epoxy.
 11. The material system of claim 10,wherein the metal substrate comprises titanium, aluminum, copper, oralloys thereof.
 12. The material system of claim 10, wherein the firstelectrode or second electrode comprises a conductive epoxy.
 13. Thematerial system of claim 10, further comprising a second coating layerdisposed on the first electrode and the first coating layer such thatthe first electrode is embedded between the first coating layer and thesecond coating layer.
 14. The material system of claim 13, furthercomprising a third electrode directly disposed on the second coatinglayer.
 15. The material system of claim 13, wherein the second coatinglayer has a thickness from about 4 μm to about 15 μm.
 16. The materialsystem of claim 10, wherein the first electrode or the second electrodehas a shape selected from the group consisting of circular, star,rectangular, pentagonal, hexagonal, heptagonal, and octagonal.
 17. Amaterial system comprising: a metal substrate; a first coating layerdisposed on the metal substrate; a first electrode disposed directly onthe first coating layer; and a second electrode disposed on the metalsubstrate or the first coating layer, wherein: the first electrode orthe second electrode has one or more spokes extending therefrom, thecoating layer comprises an epoxy, and the first electrode or secondelectrode comprises a conductive epoxy.
 18. The material system of claim17, wherein the first electrode and second electrode each comprises aconductive epoxy.
 19. The material system of claim 17, wherein: thefirst electrode and the second electrode each has a thickness from about1 μm to about 12 μm, and the second coating layer has a thickness fromabout 4 μm to about 15 μm.
 20. The material system of claim 19, furthercomprising: a second coating layer disposed on the first electrode andthe first coating layer such that the first electrode is embeddedbetween the first coating layer and the second coating layer; and athird electrode directly disposed on the second coating layer.